Critical temperature dependence of high frequency electron dynamics in superconducting hot -electron bolometer mixers

Superconducting diffusion-cooled hot-electron bolometer (HEB) mixers are ideal candidates for use in terahertz heterodyne spectroscopy (THz). This thesis investigates mixer performance as a function of the superconducting transition temperature (Tc). In the present work, three different materials systems have been used to obtain devices with Tc between 1 and 5.5 K-Al, Nb in a magnetic field, and Nb-Au. The mixing measurements were conducted in a 30 GHz microwave setup with the mixer block at T = 0.22K. Microwave measurements are used to understand the underlying physics of the HEB mixer, and are a guide for the future development of THz receivers. The mixer noise temperature is found to be equal to approximately 25 times T c, and the dominant noise is assumed to be thermal fluctuations. The LO power is found to equal (0.7 nW/K2) · Tc 2 when the HEB is cooled to well below Tc. As for saturation effects, input saturation is expected when the incident noise power is greater than (0.14 nW/K2) · Tc2, and output saturation should result for incident power greater than (27 pW/K 2) · Tc2. Additional performance limitations are observed in Al devices which have a long coherence length. Superconductivity is suppressed in the ends of the microbridge due to the proximity effect. This results in a minimum bridge length of ∼6--7 xi(0) for superconductivity to be observed, therefore imposing an upper limit for the IF bandwidth of approximately (8 GHz/K) · Tc for N-S-N HEB structures. Output saturation is also more likely in Al devices with normal ends. Nb-Au mixers perform the best and are very convenient with their tunable Tc. A mixer Tc of 2 K is predicted to be optimum for many applications. Such devices should have lower noise than Nb and should not exhibit saturation effects.